In telephone systems comprising electronic subscriber line interface circuits ("SLIC's") each wire of a subscriber's line is connected to the output of a separate amplifier, alternatively called a driver, which is part of the stage which is commonly referred to as the driver stage. In the speech condition the subscriber's line is supplied from the exchange, via the drivers, with the direct voltages prescribed by the administrations (and consequently also with a prescribed voltage difference) of 45 V or 60 V between the two wires on which voltages speech signals are superposed. Due to these speech signals and the minimum required voltage drop across the output transistors of the drivers each driver is supplied from a supply source producing a voltage difference which is somewhat larger than the said voltage difference between the two wires.
When the driver is manufactured in integrated form, the breakdown voltage achieved by the integration process used must exceed the supply voltage difference across the driver. This can be easily achieved for a voltage difference of the above-mentioned order of magnitude.
For ringing a subscriber the administrations prescribe an alternating voltage having a frequency of, for example, 50 Hz and a considerably greater amplitude than the above-mentioned supply voltage, for example 95 V. In general, an alternating voltage is applied to each one of the wires of the line, which alternating voltages have a mutually opposite polarity and an equal amplitude. The voltage swing on each of the wires is then equal to the amplitude of the alternating voltage on the line. It is general practice to apply this alternating voltage directly to a ringing signal generator bypassing the driver stage. This has the disadvantage that for each subscriber line a separate switching arrangement is necessary which is capable of switching the alternating voltages having the said amplitude. It is therefore advantageous to apply also the ringing signals to the subscriber line via the driver stage. This requires a supply source voltage which is somewhat larger than the said 95 V ringing voltage swing. It is however disadvantageous to use a supply voltage in the order of this value when integrated drivers are used, as manufacturing the integrated circuits in integrated form with a breakdown voltage of the order of magnitude of this increased supply voltage is difficult and has an adverse effect on the quality of the integrated circuit.
This drawback can be mitigated in a manner as described in the Netherlands Patent Application No. 78 10086 from which it is known to produce the large voltage swing from the output of the drivers by means of a fixed supply voltage difference which is smaller than this voltage swing, the voltage across the integrated circuit always being smaller than the breakdown voltage. This is effected by supplying the driver by means of a floating supply voltage which is locked onto the output signal of the driver stage.
This manner of supplying the drivers has the disadvantage that each driver requires at least one transformer. Transformers are undesirable since they are comparatively expensive and heavy, produce disturbing electro-magnetic stray fields and cannot be manufactured in integrated form.